Latent heat transport apparatuses have been known wherein latent heat transport is carried out by utilizing phenomena of evaporation, boiling, condensation, etc. of a working liquid sealed into the apparatuses. The latent heat transport apparatus may, for example, be a heat pipe or a double phase closed type heat siphon. The heat pipe is an apparatus which transports heat by utilizing capillary force obtained by an internally installed wig (capillary structure), and the double phase closed type heat siphon is an apparatus which transports heat by utilizing gravity and centrifugal force. They are characterized in that a working liquid can be circulated without using an external power such as a pump.
The heat pipe is applied to relatively small refrigerators such as refrigerators for semiconductor devices or electronic equipments, and since the double phase closed type heat siphon does not require a wig and has a simple structure, it is widely used for gas-gas heat exchangers or for accelerating snow melt on road, cryoprotection, etc.
Now, properties required for a working liquid will be described with reference to a heat pipe as an example.
The heat pipe is a heat transport device, wherein one terminal of a pipe container is an evaporation part, and the other terminal is a condensation part. In general, heat transport characteristics of a heat pipe depend on a working liquid contained in the main body of the heat pipe. In general, when it is used under normal pressure at about the boiling point, excellent characteristics are shown.
The mechanism of the heat pipe is simple. If one terminal of a pipe is heated, the working liquid evaporates there and absorbs heat. Evaporated gas diffuses in the pipe, emits latent heat at the other terminal (low temperature part) and is condensed to a liquid. The liquid returns to one terminal (high temperature part) by gravity or capillary force, whereby heat is transferred from the high temperature part to the low temperature part.
The heat pipe mainly comprises three elements: (1) working liquid, (2) wig or capillary structure and (3) vessel or container. Characteristics required for the working liquid are as mentioned below. However, the operation vapor temperature range is the most important.
(1) conformity to a wig or container material
(2) excellent heat stability
(3) excellent wettability to a wig or container material (contact angle is 0 or extremely low)
(4) vapor pressure is not too high or too low within the operation temperature range
(5) large evaporation latent heat
(6) high heat conductivity
(7) low viscosity in both gas and liquid phases
(8) strong surface tension
(9) appropriate freezing point or appropriate melting point
In general, as a characteristic value to determine the maximum heat transport amount, merit number M (kJ/(m2·s) represented by the following formula 2 is used. The larger the merit number is, the larger the maximum heat transport amount of the working liquid becomes.M=ρσL/μ  Formula 2,wherein ρ is the density of the working liquid (kg/m3), σ is the surface tension of the working liquid (N/m), L is the evaporation latent heat of the working liquid (kJ/kg), and μ is the viscosity of the working liquid (Pa·s).
The merit number is variable depending on the temperature.
Heretofore, as the working liquid for heat pipes, water, ammonia, methanol, a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), etc. are used. However, in the case of water, there is a problem of freezing in cold district. Whereas, in the case of ammonia, not only conformity with a heat pipe container is required, but also handling is cumbersome due to a bad smell and toxicity. In the case of methanol, there is a problem of corrosion of an aluminum or stainless steel container. Further, CFC and HCFC are inhibited compounds, since they destroy an ozone layer.
In order to solve the above problems, the following methods have been proposed.
For example, in Patent Document 1, it has been proposed to use n-perfluorohexane as a working fluid for heat pipes or heat siphons. Further, in Patent Document 2, a working liquid for heat pipes containing at least 95% of a perfluorocarbon such as C6F14 and at most 1% of a fluorocarbon compound having a lower boiling point than the perfluorocarbon has been proposed. However, heat conductivities of these working liquids are poor, and there is a problem of global warming.    Patent Document 1: JP-A-59-12288 (claims)    Patent Document 1: JP 2726542 (claims)